Monitor lizard

Monitor lizards are generally large reptiles, although some can be as small as 12 centimetres in length. They have long necks, powerful tails and claws, and well-developed limbs. Most species are terrestrial, but arboreal and semi-aquatic monitors are also known. Almost all Monitor lizards are carnivorous, although Varanus prasinus and Varanus olivaceus are also known to eat fruit. They are oviparous, laying from 7 to 37 eggs, which they often cover with soil or protect in a hollow tree stump.

Evolutionary overview

Monitor lizards differ greatly from other lizards in several ways, possessing a relatively high metabolic rate for reptiles and several sensory adaptations that benefit the hunting of live prey. Recent research indicates that the varanid lizards, including the Komodo dragon, may have very weak venom. This possible discovery of venom in Monitor Lizards, as well as in agamid Lizards, led to the Toxicofera hypothesis: that all venomous reptiles share a common venomous ancestor. Some monitor lizards are apparently capable of parthenogenesis.

During the Pleistocene, giant monitor lizards lived in Southeast Asia and Australiasia, the best known fossil being Megalania prisca. Recently reclassified as a constituent of the genus Varanus, this species is an iconic member of the Pleistocene megafauna of Australia.

Origins of the name

The genus name Varanus is derived from the Arabic word waral ورل, which is translated to English as "monitor". It has been suggested that the occasional habit of varanids to stand on their two hind legs and to appear to "monitor" their surroundings led to the original Arabic name. According to legend, these lizards were supposed to warn people that crocodiles were nearby.

Intelligence

Varanid lizards are very intelligent, and some species can even count. Careful studies feeding V. albigularis at the San Diego Zoo varying numbers of snails showed that they can distinguish numbers up to six. V. niloticus have been observed to cooperate when foraging. One varanid lures the female crocodile away from her nest while the other opens the nest to feed on the eggs. The decoy then returns to also feed on the eggs.King, Dennis & Green, Brian. 1999. Goannas: The Biology of Varanid Lizards. University of New South Wales Press. ISBN 0-86840-456-X, p. 43. Komodo dragons, V. komodoensis, at the Smithsonian National Zoological Park in Washington, D.C., recognize their keepers and seem to have distinct personalities.

In captivity

Monitor lizards have become a staple in the reptile pet trade. The most commonly kept monitors are the savannah monitor and Acklin's monitor, due to their relatively small size, low cost, and relatively calm dispositions. Nile monitors, white throated monitors,water monitors, Mangrove monitors, and emerald tree monitors have also been kept in captivity. Like all reptiles that are kept as pets, monitors need hiding places and an appropriate substrate. Monitors also need a large water dish in which they can soak their entire body. In the wild, monitors will eat anything they can overpower, but crickets, superworms, and the occasional rodent make up most of the captive monitors' diet. Boiled eggs, silkworms, earthworms, and feeder fish can also be fed to them. However, due to their predatory nature and large size some monitors can be dangerous to keep as pets; adult nile monitors and water monitors, for example can reach seven feet in length.